Plant thymidine kinases and their use

ABSTRACT

This invention relates to novel plant thymidine kinases and their use in gene therapy. More specifically the invention provides novel thymidine kinases derived from tomato, pine, rice or thale cress. In further aspects the invention provides novel polynucleotides encoding the plant thymidine kinases, vector constructs comprising the polynucleotide, host cells carrying the polynucleotide or vector, methods of sensitising cells to prodrugs, methods of inhibiting pathogenic agents in warm-blooded animals, methods for biocontrol of plants, methods of synthesizing monophosphates and pharmaceutical compositions comprising the plant thymidine kinases of the invention. In a preferred embodiment the invention provides a unique combination of a plant thymidine kinase and the nucleoside analog AZT to treat abnormal cell growth.

TECHNICAL FIELD

This invention relates to novel plant thymidine kinases and their use in gene therapy. More specifically the invention provides novel thymidine kinases derived from tomato, pine, rice or thale cress.

In further aspects the invention provides novel polynucleotides encoding the plant thymidine kinases, vector constructs comprising the polynucleotide, host cells carrying the polynucleotide or vector, methods of sensitising cells to prodrugs, methods of inhibiting pathogenic agents in warm-blooded animals, methods for biocontrol of plants, methods of synthesizing monophosphates and pharmaceutical compositions comprising the plant thymidine kinases of the invention.

In a preferred embodiment the invention provides a unique combination of a plant thymidine kinase and the nucleoside analogue AZT to treat abnormal cell growth.

BACKGROUND ART

DNA is made of four deoxyribonucleoside triphosphates, provided by the de novo and the salvage pathway. The key enzyme of the de novo pathway is ribonucleotide reductase, which catalyses the reduction of the 2′-OH group of the nucleoside diphosphates, and the key salvage enzymes are the deoxyribonucleoside kinases, which phosphorylate deoxyribonucleosides to the corresponding deoxyribonucleoside monophosphates.

Deoxyribonucleoside kinases from various organisms differ in their substrate specificity, regulation of gene expression and cellular localisation. In mammalian cells there are four enzymes with overlapping specificities, the thymidine kinases 1 (TK1) and 2 (TK2), deoxycytidine kinase (dCK) and deoxyguanosine kinase (dGK), which phosphorylate purine and pyrimidine deoxyribonucleosides. TK1 and TK2 are pyrimidine specific and phosphorylate deoxyuridine (dUrd) and thymidine (dThd), and TK2 also phosphorylates deoxycytidine (dCyd). dCK phosphorylates dCyd, deoxyadenosine (dAdo) and deoxyguanosine (dGuo), but not dThd. dGK phosphorylates dGuo and dAdo. TK1 and dCK are cytosolic, and TK2 and dGK are localised in the mitochondria, although recent reports indicate a cytoplasmic localisation of TK2 as well.

Based on homology to a thymidine kinase derived from a Myxoma virus, a gene from rice encoding a thymidine kinase has been proposed [Hemayet Ullah, Dominique Robertson, and Roger C. Fites: A Gene for Thymidine Kinase in Plants (Accession No. AF066050; Plant gene register PGR99-048) Plant Physiol. 1999 119 1567]. However, only a partial sequence was isolated, which sequence is not sufficient for expression of the active protein.

Two pieces of genomic DNA from Arabidopsis thaliana have been annotated as putative thymidine kinases in GenBank™ (Accession Nos. AAF13097 and BAB09824). However, to this date no experimental work towards characterisation, properties, localisation, use or biological function of plant kinases has yet been accomplished.

AZT (3′-azido-3′-deoxythymidine, Zidovudine, Retrovir®) is a nucleoside analog used in the treatment of HIV-infections. The rate-limiting step of its activation in human cells is the activation of AZT-monophosphate (AZTMP) to AZT-diphosphate.

It has been suggested to use human Herpes simplex virus type 1 thymidine kinase (HSV1-TK), having an endogenous thymidine monophosphate kinase activity, for gene therapy treatment of HIV-infections. HSV1-TK is phylogenetically related to human TK2 but not to human TK1. Also the use of HSV1-TK in order to improve the antiviral activity of zidovudine has been suggested, and the use of HSV1-TK in the combination with AZT has shown to be able to kill transformed E. coli bacteria. However, because it is believed that the phoshorylation of AZTMP is the rate limiting step in AZT activation in humans, no experimental work towards an effective combination of AZT and a thymidine kinase for use in the treatment of human cancer or in other human abnormal cell growth related diseases has been accomplished.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide novel plant thymidine kinases useful for converting nucleoside analogs into toxic substances, and useful for converting nucleoside analogues into monophosphates and nucleoside analogue monophosphates into the corresponding diphosphates. The isolated plant thymidine kinases genes can be used in gene therapy for selectively killing cells containing the genes (and being exposed to at least one nucleoside analogue) and the isolated thymidine kinases provided by the present invention can be used for the industrially important process of phosphorylating nucleosides and/or nucleoside analogues. The isolated thymidine kinases can also be used for killing cells by injecting the enzymes into the cells and subjecting the cells to at least one nucleoside analogue. Whenever reference is made to a nucleoside analogue and/or its monophosphate, it is understood that this nucleoside analogue is preferably a deoxynucleoside analogue, more preferably a deoxyribonucleoside analogue, and more preferably AZT.

More specifically the invention provides a unique combination of a plant thymidine kinase and the nucleoside analog AZT to treat abnormal cell growth.

Accordingly, in its first aspect, the invention provides isolated polynucleotides encoding plant thymidine kinase enzymes derived from pine (Pinus taeda), from rice (Oryza sativa) or from tomato (Lycopersicum esculentum). These plant thymidine kinases have turned out to be especially efficient in phosphorylating AZT and posess an unexpected high endogenous monophosphate kinase activity especially on nucleoside analogue monophosphates such as AZTMP.

In another aspect the invention provides isolated polynucleotides encoding thymidine kinase enzymes derived from plants, which thymidine kinase enzymes, when compared to human Herpes simplex virus type 1 thymidine kinase (HSV1-TK) and upon transduction into a eukaryotic or a bacterial cell, decreases at least four (4) fold the IC₅₀ of at least one nucleoside analogue. Such low IC₅₀ has not been reported for any plant thymidine kinase before. Preferably this nucleoside analogue is AZT.

In a third aspect the invention provides polynucleotides encoding thymidine kinase enzymes derived from plants, which thymidine kinase enzymes are capable of phosphorylating at least one nucleoside analogue monophosphate at higher degree than thymidine monophosphate. Preferably the analogue monophosphate is AZT monophosphate. More preferably said kinases are essentially not capable of phosphorylating thymidine monophosphate. Endogenous monophosphate kinase activity has not been reported for plant thymidine kinases before. Posessing this activity makes the kinases good candidates for medical use such as cancer therapy and for suicide systems. Thus, by having higher substrate specificity for AZT monophosphate than for thymidine monophosphate, AZT monophosphate is converted into the diphosphate at a higher rate.

In a fourth aspect the invention provides polynucleotides encoding thymidine kinase enzymes derived from plants, which thymidine kinase enzymes have a ratio of [k_(cat)/K_(m) (Thd)]/[k_(cat)/K_(m) (AZT)] of less than two (2). Thymidine kinases with such a low ratio are relatively more efficient in phosphorylating AZT compared to Thd than known thymidine kinases.

In a fifth aspect the invention provides polynucleotides encoding thymidine kinase enzymes derived from plants, which thymidine kinase enzymes, when expressed and compared to human Herpes simplex virus type 1 thymidine kinase (HSV1-TK), have a decreased ratio of [k_(cat)/K_(m) (Thd)]/[k_(cat)/K_(m) (at least one nucleoside analogue)] of at least five (5) fold, wherein the analogue is any nucleoside analogue. Such plant derived thymidine kinases are more effective in the treatment of cancer or other mammalian abnormal cell growth related diseases or in suicide systems than is human Herpes simplex virus type 1 thymidine kinase, which is the most promising enzyme in the prior art for such purpose.

In a sixth aspect the invention provides isolated mutated and/or truncated polynucleotides encoding thymidine kinase enzyme variants derived from plants, which thymidine kinase enzyme variants, when compared to human Herpes simplex virus type 1 thymidine kinase (HSV1-TK) and upon transduction into a eukaryotic or a bacterial cell, decrease at least four (4) fold the IC₅₀ of at least one nucleoside analogue. The mutated and/or truncated thymidine kinases have improved substrate specificity for the nucleoside analogues and/or improved stability.

In a seventh aspect the invention provides thymidine kinase enzymes expressed by a polynucleotide of the invention, or thymidine kinase enzyme variants expressed by a mutated (and/or truncated) polynucleotide of the invention.

In an eight aspect the invention provides plant thymidine kinase enzymes derived from thale cress (Arabidopsis thaliana), pine (Pinus taeda), from tomato (Lycopersicum esculentum) or from rice (Otyza saliva).

In a ninth aspect the invention provides vector constructs comprising a polynucleotide of the invention, and a promoter operably linked to the polynucleotide.

In a tenth aspect the invention provides packaging cell lines capable of producing infective virions comprising a vector of the invention.

In an eleventh aspect the invention provides host cells comprising a polynucleotide of the invention, or an expression vector of the invention.

In a twelvth aspect the invention provides pharmaceutical compositions comprising a plant thymidine kinase enzyme of the invention, the polynucleotide of the invention, or a packaging cell line of the invention, and a pharmaceutically acceptable carrier or diluent.

In a thirteenth aspect the invention provides methods of sensitising cells to prodrugs, which methods comprise the steps of (i) transfecting or transducing a cell with a polynucleotide sequence encoding a plant thymidine kinase enzyme that promotes the conversion of said prodrug into a (cytotoxic) drug, or otherwise delivering the plant thymidine kinase into the cell; and (ii) delivering a prodrug to said cell; wherein the cell is more sensitive to the (cytotoxic) drug than to the prodrug.

In a fourteenth aspect the invention provides methods of inhibiting pathogenic agents in warm-blooded animals, which methods comprise administering to such animals a polynucleotide of the invention, or a vector of the invention.

In a fifteenth aspect the invention relates to the use of the plant thymidine kinase enzymes of the invention for the phosphorylation of nucleosides or nucleoside analogs.

In a sixteenth aspect the invention provides methods of phosphorylating nucleosides or nucleoside analogs, which methods comprise the steps of (i) subjecting the nucleoside or nucleoside analog to the action of the plant thymidine kinase enzyme of the invention, and (ii) recovering the phosphorylated nucleoside or nucleoside analog.

In a seventeenth aspect the invention relates to a method of controlling or modifying growth of a plant, which plant comprises plant cells comprising a polynucleotide encoding a plant thymidine kinase enzyme of the invention, which method comprises the step of exposing the plant or plant cell to a nucleoside or nucleoside analog.

In a further aspect the invention relates to articles containing a nucleoside analogue and a plant derived thymidine kinase according to the invention or a gene coding for said plant derived thymidine kinase, or a vector comprising said gene coding for said plant derived thymidine kinase, as a combination fo the simultaneous, separate or successive administration in cancer therapy.

In a further aspect the invention relates to the medical use of the thymidine kinases of the invention, the genes encoding said thymidine kinases and vectors and host cells expressing said thymidine kinases. Furthermore the invention relates to the use of said enzymes, genes, vectors and cells for the preparation of a medicament for the treatment of a disease related to abnormal cell growth in a human being, preferably the use wherein said disease is cancer.

Other objects of the invention will be apparent to the person skilled in the art from the following detailed description and examples.

DETAILED DISCLOSURE OF THE INVENTION

Plant Thymidine Kinases

In its first aspect the invention provides novel proteins having thymidine kinase (TK) enzyme activity, and which proteins are derived from plants. More specifically the novel plant thymidine kinase enzymes are derived from seed bearing plants (Spermatophyta), in particular pine (Pinus taeda), tomato (Lycopersicum esculentum), rice (Oryza sativa), and/or from thale cress (Arabidopsis thaliana).

Also it was surprisingly found that two different thymidine kinase enzymes can be isolated from Arabidopsis thaliana, herein designated AT-TK1 a and AT-TK1 b.

In the context of the present invention, a thymidine kinase is an enzyme capable of phosphorylating thymidine but not capable of phosphorylating a purine nucleoside using the assays described in Example 3, Table 5. A thymidine kinase may or may not phosphorylate deoxycytidine.

The thymidine kinase enzymes of the invention are particularly useful for the treatment of abnormal cell growth by activating nucleoside analogs; in particular ATZ.

Based on a Clustal W (1.82) multiple amino acid sequence alignment of the tomato, pine and rice thymidine kinases, three motifs and several conserved, semi-conserved and lesser-conserved residues were identified, as shown in Table 1 below. In Table 1, alignment starts with amino acid residue no. 65 of the Rice TK1, because the first 64 amino acids do not align to the other thymidine kinases. TABLE 1 CLUSTAL W (1.82) Multiple Amino Acid Sequence Alignment Rice-TK1 ---------- ---------- ---------- ---------- -MEAQPSYP- ---------- (008) Tomato-TK1 ---------- ---------- ---------- ---------- -MAFSSSARN PVDLRNGSKN (019) AT-TK1a ---------- ---------- ---------- ---------- -MATLKASFL IKTLDSDVTG (019) Pine-TK1 ---------- ---------- ---------- ---------- -MDDSGIYT- ---------- (008) AT-TK1b MRTLISPSLA PFSLHLHKPS LFSTALRFSF SIINITPTNS PPSTISTRKL QTKATRVTSS (060) Rice-TK1 ---------- -GEIHVIVGP MFAGKTTALL RRVQVEAGTG RNVALIKSDK DNRYGLDSVV (057) Tomato-TK1 SFC------P VGEIHVIVGP MFAGKTTALL RRVNLESNDG RNVVLIKSSK DARYAVDAVV (073) AT-TK1a DFLSDLEPRG SGAVHVIMGP MFSGKSTSLL RRIKSEISDG RSVAMLKSSK DTRYAKDSVV (079) Pine-TK1 ---------- SGEIHLILGP MFAGKTTALI RKMRAEIQMG RRVVLVKSDK DTRYGLNSVV (058) AT-TK1b SSSQPLSSSS PGEIHVVVGP MFSGKTTTLL RRILAERETG KRIAIIKSNK DTRYCTESIV (120)             * :*:::** **:**:*:*: *::  *   * : :.::**.* * **  :::* Rice-TK1 THDGTKMPCW ALPELSSFQD KLGTEAYD-K VDVIGIDEAQ FFDDLHDFCC KAADRDGKIV (116) Tomato-TK1 THDGTRFPCW SLPDLSSFKQ RFGEDAYE-K VDVIGIDEAQ FFGDLYEFCC NAADFDGKII (132) AT-TK1a THDGIGFPCW ALPDLMSFPE KFGLDAYN-K LDVIGIDEAQ FFGDLYEFCC KVADDDGKIV (138) Pine-TL1 SHDGAKMPCW AVADLASFKG KLGEEAYK-Q VDVIGIDEAQ FFKDLYSFCQ VAADRDGKIV (117) AT-TK1b THDGEKYPCW SLPDLSSFKE EPGFDDYENR LDVIGIDEAQ FFGDLYEFCR EAADKEGKTV (180) :***   *** ::.:* **   ::* : *. : :********* ** **:.**   .** :** :                                    -------- --                                      Motif I Rice-TK1 VVAGLDGDYK RNKFGSVLDI IPLADSVTKL TARCELCGRR AFFTLRKTRE TKTELIGGAD (176) Tomato-TK1 VVAGLDGDYL RKSFGSVLDI IPLADTVTKL TARCELCNRR AFFTFRKTNE TETELIGGAD (192) AT-TK1a IVAGLDGDYL RRSFGAVLDI IPIADSVTKL TARCEVCGHK AFFTLRKNCD TRTELIGGAD (198) Pine-TK1 IVAGLDGDYL RKSFGSALEL IPIADSVVKL KSRCELCGKA ASFTFRKTGE RKTEVVGGAD (177) AT-TK1b IVAGLDGDFM RRRFGSVLDL IPIADTVTKL TSRCEVCGKR ALFTMRKTEE KETELIGGAE (240) :*******:  *. **:.*:: **:**:*.** .:***:*.:  * **:**. :   .**::***: ------C                     ---- ------ Motif II                   Lid region Rice-TK1 VYMPVCRQHY LDGQIVIEAT RIVLD-LEKS KVIHAFK--- --- (212) Tomato-TK1 IYMPVCRQHY VNGQSVNESA KMVLE-SHKV SNELILESPL VDP (234) AT-TK1a VYMPVCRKHY ITNHIVIKAS KKVLEDSDKA RAESCVAATI --- (238) Pine-TK1 VYMPVCRRHY VNGQIVIDTT EAVLE-SPEV QYDACAQATT TSG (219) AT-TK1b VYMPVCRSHY VCGQNVLETA RAVLD---SS NNHSVVASSL --- (277) :****** ** : .: * .:: : **:   . ------ Motif III - designates motifs (Motif I, Motif II and Motif III) * designates conserved residues : designates semi-conserved residues . designates lesser-conserved residues

Therefore, in a preferred embodiment the plant thymidine kinase enzyme of the invention comprises one or more of the following three motifs: Val Ile Gly Ile Asp Glu Ala Gln Phe Phe (Motif I) Val Ala Gly Leu Asp Gly (Motif II) Tyr Met Pro Val Cys Arg (Motif III)

In another preferred embodiment, the plant thymidine kinase enzyme of the invention comprises the Lid region:

Val A1 Lys Leu A2 A3 Arg Cys Glu A4 (Lid region), wherein A1 is selected from Thr and Val, A2 is selected from Thr and Lys, A3 is selected from Ala and Ser, and A4 is selected from Leu and Val.

In a more preferred embodiment the plant thymidine kinase enzyme of the invention comprises all of the conserved residues identified in Table 1.

Identity of Polypeptides

In another preferred embodiment the plant thymidine kinase enzyme of the invention comprises the amino acid sequence presented as SEQ ID NO: 2, as SEQ ID NO: 4, as SEQ ID NO: 5, as SEQ ID NO: 7, as SEQ ID NO: 9, as SEQ ID NO: 11, or as SEQ ID NO: 13, or an amino acid sequence that has at least 30%, preferably at least 50%, more preferred at least 70%, even more preferred at least 80%, still more preferred at least 90%, yet more preferred at least 95% identity, most preferred at least 98% identity, when determined over the entire length of the SEQ ID.

In the context of this invention “identity” is a measure of the degree of homology of amino acid sequences. In order to characterize the identity, subject sequences are aligned so that the highest order homology (match) is obtained. Based on these general principles the “percent identity” of two amino acid sequences is determined using the BLASTP algorithm [Tatiana A. Tatusova, Thomas L. Madden: Blast 2 sequences—a new tool for comparing protein and nucleotide sequences; FEMS Microbiol. Lett. 1999 174 247-250], which is available from the National Center for Biotechnology Information (NCBI) web site, and using the default settings suggested here (i.e. Matrix=Blosum62; Open gap=11; Extension gap=1; Penalties gab x_dropoff=50; Expect=10; Word size=3; Filter on). The BLAST algorithm determines the % sequence identity in a range of overlap between two aligned sequences. For the purposes of the present invention, the percent sequence identity is calculated in a range of overlap of at least 50 amino acids, more preferably at least 75 amino acids, more preferably at least 100 amino acids, the range being calculated by BLASTP under default settings.

The results of this BLASTP comparison are presented in Table 2. TABLE 2 BLASTP Comparison of Protein Sequences of Thymidine Kinases of Different Plant Origin TK Tomato Rice AT(a) AT(b) Pine 65/82/197 69/84/193 59/77/208 62/83/195 Tomato — 75/86/196 69/84/193 65/81/208 Rice — — 69/84/193 65/83/197 AT(a) — — — 60/78/215 Identities (%)/Positives (%)/length of the compared fragment Pine (Pinus taeda) thymidine kinase Tomato (Lycopersicum esculentum) thymidine kinase Rice (Oryza sativa) thymidine kinase Thale cress (Arabidopsis thaliana) thymidine kinases (a) and (b)

In a preferred embodiment the plant thymidine kinase enzyme of the invention is derived from pine, and has at least at least 60%, preferably at least 70%, more preferred at least 80%, even more preferred at least 90%, yet more preferred at least 95%, most preferred at least 98% sequence identity to the sequence presented in SEQ ID NO: 2.

In another preferred embodiment the plant thymidine kinase enzyme of the invention is derived from tomato, and has at least at least 60%, preferably at least 70%, more preferred at least 80%, even more preferred at least 90%, yet more preferred at least 95%, most preferred at least 98% sequence identity to any of the sequences SEQ ID NO: 4 or 5.

In a third preferred embodiment the plant thymidine kinase enzyme of the invention is derived from rice, and has at least at least 60%, preferably at least 70%, more preferred at least 80%, even more preferred at least 90%, yet more preferred at least 95%, most preferred at least 98% sequence identity to the sequence SEQ ID NO: 7.

In a fourth preferred embodiment the plant thymidine kinase enzyme of the invention is derived from Arabidopsis thaliana, and has at least at least 60%, preferably at least 70%, more preferred at least 80%, even more preferred at least 90%, yet more preferred at least 95%, most preferred at least 98% sequence identity to the sequence SEQ ID NO:9 or 11.

In a fifth embodiment the plant thymidine kinase enzyme of the invention is also derived from Arabidopsis thaliana, and has at least at least 60%, preferably at least 70%, more preferred at least 80%, even more preferred at least 90%, yet more preferred at least 95%, most preferred at least 98% sequence identity to the sequence SEQ ID NO: 13.

Variant Polypeptides

In a most preferred embodiment the plant thymidine kinase enzyme of the invention comprises the amino acid sequence presented as SEQ ID NO: 2, as SEQ ID NO: 4, as SEQ ID NO: 5, as SEQ ID NO: 7, as SEQ ID NO: 9, as SEQ ID NO: 11, or as SEQ ID NO: 13, or a functional analogue thereof.

In the context of this invention, the term “Functional analog” means a polypeptde (or protein) having having thymidine kinase activity and having an amino acid sequence that differs from the sequence presented as SEQ ID NO: 2, as SEQ ID NO: 4, as SEQ ID NO: 5, as SEQ ID NO: 7, as SEQ ID NO: 9, as SEQ ID NO: 11, or as SEQ ID NO: 13, at one or more amino acid positions. Such analogous polypeptides include polypeptides comprising conservative substitutions, splice variants, isoforms, homologues from other species, and polymorphisms.

As defined herein, the term “conservative substitutions” denotes the replacement of an amino acid residue by another, biologically similar residue. Examples of conservative substitutions include

-   -   (i) the substitution of one non-polar or hydrophobic residue         such as alanine, leucine, isoleucine, valine, proline,         methionine, phenylalanine or tryptophan for another, in         particular the substitution of alanine, leucine, isoleucine,         valine or proline for another, or     -   (ii) the substitution of one neutral (uncharged) polar residue         such as serine, threonine, tyrosine, asparagine, glutamine, or         cysteine for another, in particular the substitution of arginine         for lysine, glutamic for aspartic acid, or glutamine for         asparagine; or     -   (iii) the substitution of a positively charged residue such as         lysine, arginine or histidine for another, or     -   (iv) the substitution of a negatively charged residue such as         aspartic acid or glutamic acid for another.

The term conservative substitution also include the use of a substituted amino acid residue in place of a parent amino acid residue, provided that antibodies raised to the substituted polypeptide also immunoreact with the un-substituted polypeptide.

Modifications of this primary amino acid sequence may result in proteins which have substantially equivalent activity as compared to the unmodified counterpart polypeptide, and thus may be considered functional analogous of the parent proteins. Such modifications may be deliberate, e.g. as by site-directed mutagenesis, or they may occur spontaneous, and include splice variants, isoforms, homologues from other species, and polymorphisms. Such functional analogous are also contemplated according to the invention.

C-terminal Deletions

In another embodiment the invention provides plant thymidine kinase enzymes having C-terminal deletions when compared to the parent (wild-type) enzyme. Such truncated enzymes may be obtained by conventional techniques, e.g. site-directed mutagenesis, or as described in the working examples.

According to the invention it has been found that C-terminal deletions create enzymes of improved properties, in particular increased stability, and/or improved substrate specificity, when compared to the wild-type enzyme.

In a more preferred embodiment the invention provides thymidine kinase enzymes having a C-terminal deletion in the order of 1-60 amino acid residues, preferably 1-50 amino acid residues, more preferred 1-40 amino acid residues, even more preferred 1-30 amino acid residues, yet more preferred 1-26 amino acid residues, most preferred 1-24 amino acid residues.

In an even more preferred embodiment, the plant thymidine kinase enzyme of the invention is a thymidine kinase enzyme derived from tomato that has a C-terminal deletion of 26 amino acid residues. In a most preferred embodiment the plant thymidine kinase enzyme of the invention is a thymidine kinase enzyme having the amino acid sequence of SEQ ID NO: 5.

In a yet more preferred embodiment, the plant thymidine kinase enzyme of the invention is a thymidine kinase enzyme derived from Arabidopsis thaliana that has a C-terminal deletion of 24 amino acid residues. In a most preferred embodiment the plant thymidine kinase enzyme of the invention is a thymidine kinase enzyme having the amino acid sequence of SEQ ID NO: 11.

N-Terminal Deletions

N-terminal deletions are also contemplated, in particular the deletion of the N-terminal 22 amino acids and the deletion of the N-terminal 45 amino acids or of the N-terminal 63 amino acids of the AT-TK1b (SEQ ID NO: 13). These amino acids constitute a putative organelle signal peptide. As evidenced by Example 4 (Table 8) deletion of these N-terminal amino acids decreases the LD₁₀₀ for AZT significantly.

Polynucleotides Encoding Plant Thymidine Kinases

In another aspect the invention provides isolated polynucleotides encoding plant thymidine kinase enzymes derived from pine, tomato, rice or thale cress, preferably those plant thymidine kinase enzymes described above.

Hybridisation Protocol

In a preferred embodiment, the isolated polynucleotide of the invention is capable of hybridising with the polynucleotide sequence presented as SEQ ID NO: 1, as SEQ ID NO: 3, as SEQ ID NO: 6, as SEQ ID NO: 8, as SEQ ID NO: 10, as SEQ ID NO: 12, or its complementary strand.

Hybridization should be accomplished under at least low stringency conditions, but preferably at medium or high stringency conditions.

Suitable experimental conditions for determining hybridisation at low, medium, or high stringency conditions, respectively, between a nucleotide probe and a homologous DNA or RNA sequence, involves pre-soaking of the filter containing the DNA fragments or RNA to hybridise in 5×SSC [Sodium chloride/Sodium citrate; cf. Sambrook et al.; Molecular Cloning: A Laboratory Manual, 2^(nd) Ed., Cold Spring Harbor Lab., Cold Spring Harbor, N.Y. 1989] for 10 minutes, and prehybridization of the filter in a solution of 5×SSC, 5× Denhardt's solution [cf. Sambrook et al.; Op cit.], 0.5% SDS and 100 μg/ml of denatured sonicated salmon sperm DNA [cf. Sambrook et al.; Op cit.], followed by hybridisation in the same solution containing a concentration of 10 ng/ml of a random-primed [Feinberg A P & Vogelstein B; Anal. Biochem. 1983 132 6-13], ³²P-dCTP-labeled (specific activity>1×10⁹ cpm/μg) probe for 12 hours at approximately 45° C.

The filter is then washed twice for 30 minutes in 2×SSC, 0.5% SDS at a temperature of at least 55° C. (low stringency conditions), more preferred of at least 60° C. (medium stringency conditions), still more preferred of at least 65° C. (medium/high stringency conditions), even more preferred of at least 70° C. (high stringency conditions), and yet more preferred of at least 75° C. (very high stringency conditions).

The complementary nucleic acids or signal nucleic acids may be labelled by conventional methods known in the art to detect the presence of hybridised oligonucleotides. The most common method of detection is the use of autoradiography with e.g. ³H, ¹²⁵I, ³⁵S, ¹⁴C, or ³²P-labelled probes, which may then be detected using an X-ray film. Other labels include ligands, which bind to labelled antibodies, fluorophores, chemoluminescent agents, enzymes, or antibodies, which can then serve as specific binding pair members for a labelled ligand.

Identity of DNA Sequences

In another preferred embodiment, the isolated polynucleotide of the invention has at least 73%, preferably at least 75%, more preferred at least 80%, even more preferred at least 90%, yet more preferred at least 95%, most preferred at least 98% identity to the polynucleotide sequence presented as SEQ ID NO: 1, as SEQ ID NO: 3, as SEQ ID NO: 6, as SEQ ID NO: 8, as SEQ ID NO: 10, or as SEQ ID NO: 12, when determined over the entire length of the SEQ ID NO:. For the purposes of the present invention the percent sequence identity is calculated when BLASTN provides a range of overlap of at least 100 nucleotides, the range being determined under default settings. More preferably the range of overlap is at least 150 nucleotides, more preferably at least 225 nucleotides, more preferably at least 300 nucleotides.

In the context of this invention, “identity” is a measure of the degree of homology of nucleotide sequences. In order to characterize the identity, subject sequences are aligned so that the highest order homology (match) is obtained. Based on these general principles, the “percent identity” of two amino acid sequences or of two nucleic acids is determined using the BLASTN algorithm [Tatiana A. Tatusova, Thomas L. Madden: Blast 2 sequences—a new tool for comparing protein and nucleotide sequences; FEMS Microbiol. Lett. 1999 174 247-250], which is available from the National Center for Biotechnology Information (NCBI) web site, and using the default settings suggested here (i.e. Reward for a match=1; Penalty for a match=−2; Strand option=both strands; Open gap=5; Extension gap=2; Penalties bap x_dropoff=50; Expect=10; Word size=11; Filter on). The BLASTN algorithm determines the % sequence identity in a range of overlap between two aligned nucleotide sequences. For the purposes of the present invention the percent sequence identity is calculated in a range of overlap of at least 100 nucleotides, the range being determined by BLASTN under default settings. More preferably the range of overlap is at least 300 nucleotides.

The results of this BLASTN comparison are presented in Table 3. TABLE 3 BLASTN Comparison of Nucleotide Sequences of Thymidine Kinases of Different Plant Origin TK Tomato Rice AT(a) AT(b) Pine  74/194* 76/158 72/236 74/206 82/51 Tomato — 73/383 74/503 74/285 Rice — — 74/315 ⋄ AT(a) — — — 77/200 Identities (%)/length of the compared fragment *divided into an N-and a C-terminal fragment ⋄ No significant similarity Pine (Pinus taeda) thymidine kinase Tomato (Lycopersicum esculentum) thymidine kinase Rice (Oryza sativa) thymidine kinase Arabidopsis thaliana thymidine kinase (a) and (b) Analogous DNA Sequences

In its most preferred embodiment, the isolated polynucleotide of the invention comprises the polynucleotide sequence presented as SEQ ID NO: 1, as SEQ ID NO: 3, as SEQ ID NO: 6, as SEQ ID NO: 8, as SEQ ID NO: 10, or as SEQ ID NO: 12, or a functional analog thereof.

In the context of this invention, the term “functional analog” covers conservatively modified polynucleotides, and polynucleotides encoding functionally equivalent polypeptides.

In the context of this invention, the term “conservatively modified polynucleotides” refers to those nucleic acids which encode identical or essentially identical amino acid sequences, or where the nucleic acid does not encode an amino acid sequence, to essentially identical sequences.

Because of the degeneracy of the genetic code, a large number of functionally identical nucleic acids encode any given protein. For instance, the codons GCA, GCC, GCG and GCU all encode the amino acid alanine. Thus, at every position where an alanine is specified by a codon, the codon can be altered to any of the corresponding codons described without altering the encoded polypeptide. Such nucleic acid variations are “silent variations,” which are one species of conservatively modified variations. Every nucleic acid sequence herein, which encodes a polypeptide, 70 also describes every possible silent variation of the nucleic acid. One of skill will recognize that each codon in a nucleic acid (except AUG, which is ordinarily the only codon for methionine) can be modified to yield a functionally identical molecule. Accordingly, each silent variation of a nucleic acid, which encodes a polypeptide, is implicit in each described sequence.

Expression Vectors

In a third aspect the invention provides recombinant expression vectors comprising the isolated polynucleotide of the invention and a promoter operably linked to the polynucleotide.

The expression vector of the invention preferably is one suitable for carrying out expression in a eukaryotic organism.

In a more preferred embodiment the expression vector of the invention is a viral vector, in particular a Herpes simplex viral vector, an adenoviral vector, an adenovirus-associated viral vector, a lentivirus vector, a retroviral vector or a vacciniaviral vector.

Packaging Cell Lines

In a fourth aspect the invention provides packaging cell lines capable of producing an infective virion, which cell line comprises a vector of the invention.

Packaging cells refers to cells containing those elements necessary for production of infectious recombinant vira, which are lacking in a recombinant virus vector. Methods for preparation of packaging cell lines are known in the prior art.

Host Cells

In a fifth aspect the invention provides an isolated host cell comprising the isolated polynucleotide of the invention, or the expression vector of the invention.

In a preferred embodiment the host cell of the invention is an a bacterial cell, preferably a eukaryotic cell, in particular a mammalian cell, a human cell, an oocyte, or a yeast cell.

In a more preferred embodiment the host cell of the invention is a human cell, a dog cell, a monkey cell, a rat cell, a pig cell, or a mouse cell.

Pharmaceutical Compositions

In a sixth aspect the invention relates to novel pharmaceutical compositions comprising a therapeutically effective amount of the plant thymidine kinase enzyme of the invention, or the host cell of the invention, and a pharmaceutically acceptable carrier or diluent.

For use in therapy the plant thymidine kinase enzyme of the invention may be administered in any convenient form. In a preferred embodiment, plant thymidine kinase enzyme of the invention is incorporated into a pharmaceutical composition together with one or more adjuvants, excipients, carriers and/or diluents, and the pharmaceutical composition prepared by the skilled person using conventional methods known in the art.

Such pharmaceutical compositions may comprise plant thymidine kinase enzyme of the invention, or antibodies against a plant thymidine kinase. The composition may be administered alone or in combination with one or more other agents, drugs or hormones.

The pharmaceutical composition of this invention may be administered by any suitable route, including, but not limited to oral, intravenous, intramuscular, inter-arterial, intramedullary, intrathecal, intraventricular, transdermal, subcutaneous, intraperitoneal, intranasal, anteral, topical, sublingual or rectal application, buccal, vaginal, intraorbital, intracerebral, intracranial, intraspinal, intraventricular, intracisternal, intracapsular, intrapulmonary, transmucosal, or via inhalation.

Further details on techniques for formulation and administration may be found in the latest edition of Remington's Pharmaceutical Sciences (Maack Publishing Co., Easton, Pa.).

The active ingredient may be administered in one or several doses per day. Currently contemplated appropriate dosages are between 0.5 ng to about 50 μg/kg thymidine kinase/kg body weight per administration, and from about 1.0 ng/kg to about 100 μg/kg daily.

The dose administered must of course be carefully adjusted to the age, weight and condition of the individual being treated, as well as the route of administration, dosage form and regimen, and the result desired, and the exact dosage should of course be determined by the practitioner.

In further embodiments, the plant thymidine kinase of the invention may be administered by genetic delivery, using cell lines and vectors as described below under methods of treatment.

Therefore, in another preferred embodiment, the invention provides pharmaceutical compositions comprising the polynucleotide of the invention, or a vector of the invention, or a packaging cell of the invention, or a host cell of the invention, and a pharmaceutically acceptable carrier or diluent.

To generate such therapeutic cell lines, the polynucleotide of the invention may be inserted into an expression vector, e.g. a plasmid, virus or other expression vehicle, and operatively linked to expression control sequences by ligation in a way that expression of the coding sequence is achieved under conditions compatible with the expression control sequences.

Suitable expression control sequences include promoters, enhancers, transcription terminators, start codons, splicing signals for introns, and stop codons, all maintained in the correct reading frame of the polynucleotide of the invention so as to permit proper translation of mRNA. Expression control sequences may also include additional components such as leader sequences and fusion partner sequences.

Methods of Treatment

The present invention, which relates to polynucleotides and proteins, polypeptides, peptide fragments or derivatives produced therefrom, as well as to antibodies directed against such proteins, peptides or derivatives, may be used for treating or alleviating a disorder or disease of a living animal body, including a human, which disorder or disease is responsive to the activity of a cytotoxic agent.

The disorder, disease or condition may in particular be a cancer or a viral infection.

Cancer cells are rapidly proliferating cells with the ability to invade and metastatize adjacent tissue and hence compromise the function of essential body parts leading to severe illness and death. The usual treatments are surgery, radiation and chemotherapy, and recently a novel treatment, gene-therapy, has emerged. In gene directed enzyme prodrug therapy (GDEPT) a gene is delivered to the cancer cells, and expressed there. The corresponding enzyme later converts the administered prodrug to an active form, arresting cell proliferation. One of the GDEPT systems is based on deoxyribonucleoside kinases and different prodrugs, nucleoside analogues.

The polynucleotides of the present invention may in particular be used as a “suicide gene”, i.e. a drug-susceptibility gene. Transfer of a suicide gene to a target cell renders the cell sensitive to compounds or compositions that are relatively non-toxic to normal cells.

Therefore, in a seventh aspect, the invention provides a method for sensitising target cells to prodrugs, which method comprises the steps of

-   -   (i) transfecting or transducing the target cell with a         polynucleotide sequence encoding a plant thymidine kinase enzyme         that promotes the conversion of said prodrug into a (cytotoxic)         drug, or otherwise delivering the plant thymidine kinase into         the cell; and     -   (ii) delivering said prodrug to said target cell;     -   wherein said target cell is more sensitive to said (cytotoxic)         drug than to said prodrug.

The target cell may be any cell of interest, in particular a human cell, a dog cell, a monkey cell, a rat cell, a cat cel, a pig cell, or a mouse cell. Preferably the target cell is a human cell.

In its broadest aspect any plant thymidine kinase enzyme may be used. However, in a preferred embodiment, the polynucleotide sequence encoding a plant thymidine kinase enzyme is a polynucleotide sequence of the invention.

In a more preferred embodiment the prodrug is a nucleoside analogue.

In the context of this invention a preferred nucleoside analogue for use according to the invention is selected from the group consisting of aciclovir (9-[2-hydroxy-ethoxy]-methyl-guanosine), buciclovir (9-(3,4-dihydroxybutyl)guanine), famciclovir (2-[2-(2-Amino-9H-purin-9-yl) ethyl]-1,3-propanediol diacetate), ganciclovir (9-[2-hydroxy-1-(hydroxymethyl)ethoxyl-methyl]-guanosine), penciclovir, valcidovir, trifluorothymidine, AZT (3′-azido-3′-deoxythymidine), AIU (5′-iodo-5′-amino-2′,5′-dideoxyuridine), ara-A (adenosine-arabinoside; Vivarabine), ara-C (cytidine-arabinoside), ara-G (9-beta-D-arabinofuranosylguanine), ara-T, 1-beta-D-arabinofuranosyl thymine, 5-ethyl-2′-deoxyuridine, 5-iodo-5′-amino-2,5′-dideoxyuridine, 1-[2-deoxy-2-fluoro-beta-D-arabino furanosyl]-5-iodouracil, idoxuridine (5-iodo-2′deoxyuridine), fludarabine (2-Fluoroadenine 9-beta-D-Arabinofuranoside), gencitabine, 3′-deoxyadenosine (3-dA), 2′,3′-dideoxyinosine (ddI), 2′,3′-dideoxycytidine (ddC), 2′,3′-dideoxythymidine (ddT), 2′,3′-dideoxyadenosine (ddA), 2′,3′-dideoxyguanosine (ddG), 2-chloro-2′-deoxyadenosine (2CdA), 5-fluorodeoxyuridine, BVaraU ((E)-5-(2-bromovinyl)-1-beta-D-arabinofuranosyluracil), BVDU (5-bromovinyl-deoxyuridine), FIAU (1-(2′-deoxy-2′-fluoro-beta-D-arabinofuranosyl)-5-iodouracil), 3TC (2′-deoxy-3′-thiacytidine), dFdC gemcitabine (2′,2′-difluorodeoxycytidine), dFdG (2′,2′-difluorodeoxyguanosine), 5-fluorodeoxyuridine (FdUrd), d4T (2′,3′didehydro-3′-deoxythymidine), ara-M. (6-methoxy purinearabinonucleoside), ludR (5-Jodo-2′ deoxyuridine), CaFdA (2′-chloro-2′-5 ara-fluoro-deoxyadenosine), ara-U (1-beta-D-arabinofuranosyluracil), FBVAU (E)-5-(2′-bromovinyl)-1-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)uracil, FMAU 1-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)-5-methyluracil, FLT 3′-fluoro-2′-deoxythymidine, 5-Br-dUrd 5-bromodeoxyuridine, 5-Cl-dUrd 5-chlorodeoxyuridine or dFdU 2′,2′-difluorodeoxyuridine.

In a more preferred embodiment the nucleoside analog for use according to the invention is AZT (3′-azido-3′-deoxythymidine).

The thymidine kinase enzyme invention may be used directly via e.g., injected, implanted or ingested pharmaceutical compositions to treat a pathological process responsive to the thymidine kinase enzyme.

The polynucleotide of the invention, including the complementary sequences thereof, may be used for the expression of the thymidine kinase enzyme of the invention. This may be achieved by cell lines expressing such proteins, peptides or derivatives of the invention, or by virus vectors encoding such proteins, peptides or derivatives of the invention, or by host cells expressing such proteins, peptides or derivatives. These cells, vectors and compositions may be administered to treatment of target areas to affect a disease process responsive to cytotoxic agents.

Suitable expression vectors may be a viral vector derived from Herpes simplex, adenovira, lentivira, retrovira, or vaccinia vira, or from various bacterially produced plasmids, and may be used for in vivo delivery of nucleotide sequences to a whole organism or a target organ, tissue or cell population. Other methods include, but are not limited to, liposome transfection, electroporation, transfection with carrier peptides containing nuclear or other localising signals, and gene delivery via slow-release systems. Specific cells may be targeted by using cell surface markers, such as markers specific for cancer cells. Both the protein and a vector construct can be targeted to cells using cell surface markers. Dividing cells may be targeted by transducing with a retroviral vector, which only infects dividing cells.

In still another aspect of the invention, “antisense” nucleotide sequences complementary to the nucleotide of the invention or portions thereof, may be used to inhibit or enhance thymidine kinase enzyme expression.

In another preferred embodiment the invention provides methods for inhibiting pathogenic agents in warm-blooded animals, which methods comprises the step of administering to said animal a polynucleotide of the invention, or an expression vector of the invention.

In a more preferred embodiment the polynucleotide sequence or the expression vector is administered in vivo.

In another preferred embodiment the pathogenic agent is a virus, a bacterium or a parasite, or even a tumour cell.

In another preferred embodiment the pathogenic agent is an autoreactive immune cell.

In an even more preferred embodiment the method further comprises the step of administering a nucleoside analogue to said warm-blooded animal.

Preferably the nucleoside analogue is selected from those described above.

In a most preferred embodiment the nucleoside analog for use according to the invention is AZT (3′-azido-3′-deoxythymidine).

Suicide Systems

One very important use of the thymidine kinase encoding genes of the present invention is for suicide systems in cell and gene based therapy. In all types of cell and gene therapy on mammals there is a need to have systems which enable the irreversible killing of transplanted cells or cells which have been transduced by the gene therapy.

There are basically two types of cell based therapies which both can benefit from having a built-in suicide system based on the thymidine kinases according to the present invention. In replacement cell therapy, naked cells are transplanted into a subject to replace cells that have lost the ability to fulfil their function in the body or to replace dead cells. Once these cells have been transplanted and are fully integrated into the body of the subject they cannot easily be removed by surgical means. By having a built-in suicide system in which a thymidine kinase of the present invention is expressed constitutively or inducibly, the cells can be killed by administering to the individual a therapeutically effective amount of a nucleoside analog, such as AZT. The nucleoside analogue can be administered if the transplanted cells start to proliferate in an uncontrolled manner. One may also wish to terminate the treatment simply because there is no need for the replacement cells anymore or because further treatment is by some other route.

The other type of cell-based therapy includes therapeutic cells which are transplanted into the body to secrete e.g. a growth factor in a certain location. Often such therapeutic cells are encapsulated and can relatively easily be removed from the body again but the incorporation of a suicide system is preferred because the cells can be killed selectively without the use of surgery.

In in vivo gene therapy the same considerations apply as with replacement cell therapy. The incorporation of a suicide gene can be achieved by construction a viral vector comprising both the therapeutic gene and a thymidine kinase according to the present invention. Preferably the therapeutic gene and the thymidine kinase is inserted under the control of the same promoter, optionally by separating them with an IRES construct.

In the cases where transplanted cells have been conditionally immortalised before transplantation there is a theoretical risk that the oncogene initiates transcription after transplantation and that the transplanted cells consequently become tumorigenic. Means to control this situation are made available by the present invention. Whenever cells are immortalised by transduction with an oncogene under the control of an inducible promoter (e.g. the Tet on-off system, the Mx1 promoter or the like), a thymidine kinase is inserted into the vector construct under the control of the same promoter (or using an IRES construct). This ensures that whenever the oncogene is transcribed, the thymidine kinase is also transcribed and the transduced and tumorigenic cells can be selectively killed by administering a nucleoside analogue, such as AZT.

Method of Phosphorylating Nucleosides

The thymidine kinase enzyme of the invention may find different utility, including both therapeutic and biotechnological applications.

In an eight aspect the invention relates to use of the plant thymidine kinase enzyme of the invention for phosphorylating nucleosides or a nucleoside analogs.

In a preferred embodiment the invention provides a method for phosphorylating a nucleoside or a nucleoside analog, comprising the steps of

-   -   i) subjecting the nucleoside or nucleoside analog to the action         of the plant thymidine kinase enzyme of the invention; and     -   ii) recovering the phosphorylated nucleoside or nucleoside         analog.

In particular, the thymidine kinases of the present invention may be used for phosphorylating dideoxyguanosine.

Genetically Modified Plants

The thymidine kinase enzyme of the invention may also find utility in methods for modifying or controlling plant growth. Therefore, in a further aspect, the invention relates to a method of controlling or modifying growth of a plant, which plant comprises plant cells comprising a polynucleotide encoding a plant thymidine kinase enzyme of the invention, which method comprises the step of exposing the plant or plant cell to a nucleoside or nucleoside analog, preferably AZT. By the discovery of hitherto unknown properties of plant thymidine kinases and in particular those described in the present invention, the inventors contemplate the use of nucleoside analogues as herbicides. As plant thymidine kinases convert nucleoside analogues at a very high rate into toxic substances, nucleoside analogues can be used as herbicides for plants having these thymidine kinases either by nature or as a heterologous gene. By inserting the thymidine kinases of the present invention into plants not having a thymidine kinase with these properties, the plant is rendered susceptible to nucleoside analogs in particular susceptible to AZT.

The polynucleotide encoding plant thymidine kinase enzyme of the invention preferably is a heterologous polynucleotide, and the plant subjected to the method of the invention preferably is a transgenic plant.

Therefore, in a yet further aspect, the invention provides transgenic plants comprising an expressible heterologous nucleic acid encoding the plant thymidine kinase enzyme of the invention, wherein the heterologous nucleic acid is introduced into the transgenic plant, or an ancestor of the transgenic plant.

The transgenic plant may be obtained by known techniques for producing genetically modified plants, e.g. by introducing into a plant cell an expression vector of the invention.

Any plant thymidine kinase may also be knocked out and/or functionally replaced by other thymidine kinase, said other kinase not being able to convert a certain nucleoside analog (for example in crop plants). The nucleoside analogue may then be used to kill weed around this genetically modified plant.

Any transformed plant obtained according to the invention can be used in a conventional breeding scheme or in vitro plant propagation to produce more transformed plants with the same characteristics and/or can be used to introduce the same characteristics in other varieties of the same or related species. Such plants are also part of the invention. Seeds obtained from the transformed plants contain the same genetical characteristics and also form part of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is further illustrated by reference to the accompanying drawing, in which:

FIG. 1 shows the dTMP kinase activity (CPM) over time (0 to 120 minutes) of AT-TK1a (▪), AT-TK1b (●) and HSV1-TK (▾), respectively; and

FIG. 2 shows AZT monophosphate kinase activity (CPM) (0 to 120 minutes) of AT-TK1a (□), AT-TK1 b (◯) and HSV1-TK (∇), respectively.

EXAMPLES

The invention is further illustrated with reference to the following examples, which are not intended to be in any way limiting to the scope of the invention as claimed.

Example 1

Construction of a Retrovirus Vector Expressing Plant Kinases

The cDNA of plant kinases were cloned into a retrovirus vector based oh the Moloney murine leukemia (MLV) virus to generate a replicafion-deficient recombinant retrovirus containing the TK1 kinases.

DNA fragments were amplified with Pfu polymerase (Stratagene) using primers with designed flanking restriction enzyme sites.

Arabidopsis constructs based on AT-TK1a were cut with BamHI/XhoI, and tomato PCR fragments were cut with BglII/XhoI, and cloned into the BglII-XhoI site of plasmid vector pLCXSN, which is a retroviral transfer vector derived from pLXSN (Clontech, cat# K1060-B) by insertion of the CMV promoter upstream of the polylinker.

Four constructs were obtained: AtTK1 (PZG53), AtTK1ΔC24 (PZG56), TomTK1 (PZG69) and TomTK1ΔC26 (PZG59). pLCXSN alone and the vector containing HSV1-TK (cloned into BamHI/XhoI site) were used as a control.

The plasmids were purified using the Qiagen plasmid kit (QIAGEN) and DNA sequences of the constructed plasmids were verified by DNA sequence determination.

The following primer sequences were used: (AT TK1-for; SEQ ID NO: 14) 5′ ccg ctc gag atg gcg act ctc aaa gct tcc ttt ttg 3′; (AT TK1-rev; SEQ ID NO: 15) 5′ cgc gga tcc tta gat tgt agc agc aac aca gga ttc 3′; (AT TK1 DC; SEQ ID NO: 16) 5′ cgc gga tcc tta aac aat atg att agt gat gta atg ctt g 3′; (T TK1 DC; SEQ ID NO: 17) 5′ gga aga tct tta gac aga ttg tcc att aac ata gtg ctg 3′; (T TK1-rev; SEQ ID NO: 18) 5′ gga aga tct tta tgg atc aac tag tgg tga ttc taa g 3′; and (T TK1-for; SEQ ID NO: 19) 5′ ccg ctc gag atg gct ttt tca tca tct gct aga aac 3′.

For comparison an expression plasmid for human Herpes simplex virus type 1 thymidine kinase (HSV1-TK) was also constructed. The thymidine kinase from human HSV1 was amplified using the primers: (HSV1-TK-for; SEQ ID NO: 20) 5′ tat agg atc cgc cac cat ggc ttc gta ccc cgg c 3′; and (HSV1-TK-rev; SEQ ID NO: 21) 5′ tat act cga gga ggt cga ctc agt tag cc 3′;

-   -   and using the plasmid pCMV-pacTK described by Karreman [Karreman         C; Gene 1998 218 57-62] as template.

293T packaging cells (ATTC CRL-11268) were cultured at 37° C. in OPTIMEM 1 medium (Life Technologies, Inc.). The constructed pLCXSN plasmid vector was transfected into the packaging cells using LipofectAMINE PLUS (Life Technology Inc.) according to the protocol provided by the supplier. The medium from the transfected cells was collected 48 hours after transfection, filtered through a 0.45 mm filter, pelleted by ultracentrifugation (50,000×g, 90 minutes at 4° C.) and dissolved in TEN buffer (100 mM NaCl, 10 mM Tris pH 7.5, 1 mM EDTA).

The virus-containing buffer was subsequently used to transduce the cancer cell lines with MOI of 5.

Cell Culture and Retroviral Transduction

Glioblastoma U-87 MG (ATCC HTB-14), and glioblastoma U-118 MG (ATCC HTB-15) cells were purchased from the American Type Culture Collection. All cells were cultured in Minimum essential medium, E-MEM M (Bio Whittaker Cat. No. 12-611) with 10% (v/v) Australian originated fetal calf serum (Bio Whittaker Cat. No. 14-701) and 1 ml/l of gentamicin (Bio Whittaker Cat. No. 17-518 L). Cells were grown at 37° C. in a humidified incubator with a gas phase of 5% CO₂.

The cells lines were transduced with the retrovirus containing medium mixed with 5 μg/ml Polybrehe, incubated for 48 hours and then cultured continuously for 3 weeks in the presence of 200 μg/ml geneticin (Life Technologies Inc.).

Cell Proliferation Assays

Cells were plated at 1500-2500 cells/well in 96-well plates coated with poly-L-lysine. AZT (3′-azido-3′-deoxythymidine; available from Sigma) was added after 24 hours, and the medium containing the nucleoside analogs was not changed.

Cell survival was assayed by the XTT assay (Roche) after 5 days of drug exposure.

Each experiment was performed in four replicates. The IC₅₀ value of the investigated compounds was calculated as the mean value of these experiments using SigmaPlot® (Dyrberg Trading, Karlslunde, D K) and calculated according to the expression: d _(I)=Max/(1+([I]/IC ₅₀))

-   -   wherein     -   d_(I)=cell growth at inhibitor concentration as determined by         the XTT assay;     -   Max=maximal cell growth as determined by the XTT assay;     -   [I]=the inhibitory concentration; and     -   IC₅₀=(50% growth inhibitory concentration) a dose that inhibits         cell growth by 50%.         Expression of Plant Kinases of Increased Sensitivity to AZT

The sensitivity of the untransduced cells, and of the cells transduced with either the retroviral vector alone or the vector containing plant kinases for AZT, was determined.

The cytotoxicity (IC₅₀) was determined after 5 days of drug exposure. The results of this determination are presented in Table 4. TABLE 4 Sensitivity (IC₅₀) of glioblastoma cell lines to AZT (mM) The concentrations which cause 50% lethality are shown for the each construct and parental cell line. The factor of sensitivity increase is compared to the parental cell line. IC₅₀ None PZG53 PZG56 PZG59 PZG69 HSV1-TK pLCXSN Transient polyclonal cell lines U-87 MG 4.468 0.097 0.303 0.026 0.047 >5 >5 U-118 MG 1.021 0.062 0.296 0.034 0.045 1.923 0.859 Stable polyclonal cell lines U-87 MG 2.439 0.0719 0.1324 0.0251 0.017 2.577 4.0676 U-118 MG 0.779 0.095 0.2027 0.0266 0.1902 2.7264 2.3335

The difference in sensitivity between the parental cell lines and the cells transduced with the pLCXSN vector alone was less than 1-fold. Both glioblastoma cell lines, that expressed plant kinases, showed an increase in sensitivity to AZT. The highest increase was detected for the U-87 MG cell line expressing tomato TK1 with an almost 240-fold decrease in IC₅₀ compared with the untransduced cells.

Example 2

Cloning of Thymidine Kinases

This example describes how the genes encoding the tomato, pine, rice and thale cress thymidine kinases of the invention were identified, and how vectors to express various thymidine kinases were constructed.

Based on their homology to the human TK1, two sequences from tomato, ACCN BE463259 and ACCN BG129197 (available from the Clemson University Genomics Institute, USA), a sequence from pine, ACCN AW755132 (available from Dr. Ross Whetten, North Carolina State University, USA), and an EST, ACCN D24903 (available from the MAFF DNA Bank, 1-2,2-chome, Kannondai, Tsukuba, Ibaraki 305-8602, Japan) with homology to the postulated TK1 from rice (ACCN AF066050) were identified using the local homology search tool (tBLASTn) available from the NCBI web-site, and using the standard settings (i.e. Filter on, Expect=10, Word size=3, Matrix=Blosum62, Gap costs=Existence 11 Extension 1).

Starting from plasmid-derived primers the inserts were sequenced. Subsequently primers were designed based on the newly obtained sequence data. The sequence of the insert in D24903 combined with sequence information derived from ACCN AU068889 predicted an ORF for a 64 amino acids longer protein than reported in AF066050.

Tomato Thymidine Kinase

The ORF of the tomato thymidine kinase was amplified by PCR using the following primers: (1MS TOTK1-B; SEQ ID NO: 22) 5′ CGC GGA TCC ATG GCT TTT TCA TCA TCT GCT AGA AAC CCA GTT GAC CTG AG 3′; and (2MS TOTK1-E; SEQ ID NO: 23) 5′ CCG GAA TTC TTA TGG ATC AAC TAG TGG TGA TTC TAA G 3′; and

-   -   using the plasmid containing ACCN BG129197 as the template.

The PCR fragment was subsequently cut by EcoRI/BamHI and ligated into pGEX-2T vector (Amersham-Pharmacia), that was also cut by EcoRI/BamHI. The resulting plasmid was named pGEX-2T-TOM-TK1

Arabidopsis thaliana Thymidine Kinase (AT-TK1 a)

The ORF of TK1 a from Arabidopsis thaliana (Accession No. MF13097) was amplified from a cDNA library (Stragene) using the following primers: (1msAtTK1-B; SEQ ID NO: 24) 5′ CGC GGA TCC ATG GCG ACT CTC AAA GCT TCC TTT TTG ATC AAA ACC C 3′; and (2msAtTK1-E; SEQ ID NO: 25) 5′ CCG GAA TTC TTA GAT TGT AGC AGC AAC ACA GGA TTC AGC 3′.

The PCR fragment was subsequently cut by EcoRI/BamHI and ligated into pGEX-2T vector (Amersham-Pharmacia) that was also cut by EcoRI/BamHI. The resulting plasmid was named pGEX-2T-AT-TK1a.

Arabidopsis thaliana Thymidine Kinase (AT-TK1 b)

The ORF of TK1b from Arabidopsis thaliana (Accession No. BAB09824) was amplified from a cDNA library (Stragene) using the following strategy, which also created several N-terminal deleted mutants of AT-TK1b.

The full length ORF (ACCN BAB09824) was amplified from a cDNA library (Stratagene) using the primers: (1mtAtTK1L; SEQ ID NO: 26) 5′ ATG AGA ACA TTA ATC TCA CCA TCT C 3′; and (2mtAtTK1L; SEQ ID NO: 27) 5′ CTA AAG TGA ACT TGC TAC AAC ACT ATG 3′.

This full length PCR product was used for amplification of a fragment with BamHI/EcoRI overhangs using the primers (1mtAtTK1L-B; SEQ ID NO: 28) 5′ CGC GGA TCC ATG AGA ACA TTA ATC TCA CCA TCT C 3′; and (2mtAtTK1-E; SEQ ID NO: 29) 5′ CCG GAA TTC CTA AAG TGA ACT TGC TAC AAC AC 3′.

The PCR fragment was subsequently cut by EcoRI/BamHI and ligated into GEX-2T vector that was also cut by EcoRI/BamHI. The resulting plasmid was named GEX-2T-AT-TK1b (P661).

In analogy N-terminal deletions were made and ligated into pGEX-2T. An N-terminal deletion of 22 amino acids was achieved by using the primers (1mtAtTK1I-B; SEQ ID NO: 30) 5′ CGC GGA TCC TCC ACC GCT CTT CGC TTC TCC 3′; and

-   -   2mtAtTK1-E (SEQ ID NO: 29).

The resulting plasmid was named pGEX-2T-AT-AN22TK1b (P662).

An N-terminal deletion of 45 amino acids was achieved by using the primers (1mtAtTK1-B; SEQ ID NO: 31) 5′ CGC GGA TCC TCC ACC AGA AAG CTA CAA ACG 3′; and

-   -   2mtAtTK1-E (SEQ ID NO: 29).

The resulting plasmid was named pGEX-2T-AT-AN45TK1b (P663).

An N-terminal deletion of 63 amino acids was achieved by using the primers (1mtATTK1S-B; SEQ ID NO: 32) 5′ CGC GGA TCC CAG CCG CTC TCC TCC TCA TC 3′; and

-   -   2mtAtTK1-E (SEQ ID NO: 29).

The resulting plasmid was named pGEX-2T-AT-ΔN63TK1b (P664).

Rice Thymidine Kinase

The thymidine kinase from rice was amplified using the primers (1osTK1s-B; SEQ ID NO: 33) 5′ CGG GAT CCG GCG GCG GCG GCG GAC AAG TCT CG 3′; and (2osTK1-E; SEQ ID NO: 34) 5′ CGG AAT TCT TAC TTG AAA GCA TGG ATA ACC TTG G 3′;

-   -   and using D24903 as a template.

The PCR fragment was subsequently cut by EcoRI/BamHI and ligated into pGEX-2T vector (available from Amersham-Pharmacia) that was also cut by EcoRI/BamHI. The resulting plasmid was named pGEX-2T-Rice-TK1.

HSV1 Thymidine Kinase (Used for Control)

The thymidine kinase from HSV1 was amplified using the primers (HSV1-for A; SEQ ID NO: 35) 5′ CGC GGA TCC ATG GCT TCG TAC CCC GGC CAT C 3′; and (HSV1-rev; SEQ ID NO: 36) 5′ CCG GAA TTC TTA GTT AGC CTC CCC CAT CTC CCG 3′;

-   -   and using the plasmid pCMV-pacTK described by Karreman [Kafreman         C; Gene 1998 218 57-62] as template.

The PCR fragment was subsequently cut by EcoRI/BamHI and ligated into pGEX-2T vector (Amersham-Pharmacia) that was also cut by EcoRI/BamHI.

The resulting plasmid was named pGEX-2T-HSV1-TK.

Example 3

Expression and Purification of Recombinant Thymidine Kinases

This example describes how KY895 were transformed with the plasmids obtained according to Example 2, in order to express thymidine kinases.

KY895 cells were transformed by the expression plasmids of Example 2 using standard techniques, e.g. as described by e.g. Sambrook et al. [Sambrook et al.; Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Lab., Cold Spring Harbor, N.Y. 1989].

Transformed KY895 were grown to an OD600 nm of 0.5-06 in LB/Ampicillin (100 μg/ml) medium at 37° C. and protein expression was induced by addition of IPTG to make 100 μM. The cells were further grown for 4 h at 25° C. and subsequently harvested by centrifugation. The cell pellet was subjected to sonification in the binding buffer A (20 mM NaPO₄ pH 7,3; 150 mM NaCl; 10% Glycerol; and 0.1% Triton X-100) in presence of a protease inhibitor cocktail (Complete™—EDTA free from Roche Dignostics).

The ability of the extracts to phosphorylate the four natural deoxyribonucleoside was tested at a fixed concentration of 100 μM of the deoxyribonucleosides. The highest specific activity in each extract was set to 100%. In (parenthesis) the specific activity corresponding to 100% is given in mU/mg.

The results of these evaluations are presented in Table 5. TABLE 5 Deoxyribonucleoside Kinase Activity in Extracts of KY895 Cells 4 hours of induction with 0.1 mM IPTG at 25° C. KY895 transformed with Thd dAdo dGuo dCyd pGEX-2T n.d. n.d. n.d. n.d. pGEX-2T-TOM-TK1 100 (124) n.d. n.d.   0.1 pGEX-2T-AT-TK1a 100 (53) n.d. n.d. n.d. pGEX-2T-AT-TK1b 100 (4.4) n.d. n.d. 4 pGEX-2T-AT-ΔN22TK1b 100 (4.4) n.d. n.d. 4 pGEX-2T-AT-ΔN45TK1b 100 (5.2) n.d. n.d. 3 pGEX-2T-AT-ΔN63TK1b 100 (4) n.d. n.d. 1 n.d. designates “not detectable”.

The data in this table show that all enzymes are thymidine kinases.

The extracts were subjected to centrifugation at 10,000×g for 30 minutes, filtered, and loaded onto the column. Thereafter two 1 ml Glutathione-Sepharose columns (available from Pharmacia) were combined and equilibrated in binding buffer A. After loading of the sample, the column was washed with 40 ml of binding buffer A. Subsequently the column was washed with 5 ml 10 mM ATP/MgCl₂ in (A) and incubated for 1 hour at room temperature, and then 30 minutes at 4° C. ATP/MgCl₂ was removed by washing with 10 ml binding buffer A.

The GST-tag protein is cleaved from the TK on the GST binding column by thrombin cleavage according to the manufacturers protocol (Pharmacia).

The kinetic constants of the purified TK were determined as described by Munch-Petersen et al. [Munch-Petersen B, Knecht W, Lenz C, Søndergaard L, Pi{hacek over (s)}kur J: Functional expression of a multisubstrate deoxyribonucleoside kinase from Drosophila melanogaster and its C-terminal deletion mutants; J. Biol. Chem. 2000 275 6673-6679].

Kinetic data were evaluated by nonlinear regression analysis using the Michaelis-Menten equation v=V_(max)×[S]/(K_(m)+[S]) or Hill-equation v=V_(max)×[S]^(h)/(K_(0.5) ^(h)+[S]^(h)) as described by Knecht et al. [Knecht W, Bergjohann U, Gonski S, Kirschbaum B & Löffler M: Functional expression of a fragment of human dihydroorotate dehydrogenase by means of the baculovirus expression vector system and kinetic investigation of the purified recombinant enzyme; Eur. J. Biochem. 1996 240 292-301]. K_(m) is the Michaelis constant, K_(0.5) defines the value of the substrate concentration [S] at which v=0.5 V_(max) and h is the Hill coefficient [Cornish-Bowden A: Fundamentals of enzyme kinetics; Portland Press Ltd., London, 1995, pp. 33; and Liebecg C: IUBMB Biochemical nomenclature and related documents; Portland Press Ltd., London, 1992]. TABLE 6 Relation between Velocity and Substrate Concentration for Thd for Recombinant (r) Plant Kinases. K_(m) V_(max) k_(cat) k_(cat)/K_(m) (μM) (mU/mg) (s⁻¹) (M⁻¹ s⁻¹) rTOM-TK1 0.8  60 0.026 32500 rAT-ΔN45TK1b* 18 165 0.071 3944 rAT-TK1a* 85 452 0.2 2353 rHSV1-TK** 0.38 — 0.46 1200000 *Data from Le Breton C: In-vitro study of novel deoxyribonucleoside kinases for gene therapy; B. Sc. Report, 2002, University of Paris VII/Technical University of Denmark. **Data from Kokoris M S and Black M E: Characterization of Herpes Simplex Virus type 1 thymidine kinase mutants engineered for improved ganciclovir or acyclovir activity; Protein Science 2002 11 2267-2272.

TABLE 7 Relation between Velocity and Substrate Concentration for AZT for Recombinant Plant Kinases K_(m) V_(max) k_(cat) k_(cat)/K_(m) (μM) (mU/mg) (s⁻¹) (M⁻¹ s⁻¹) rTOM-TK1 22.4 1579 0.681 30400 rAT-ΔN45TK1b 1.46 242 0.104 71200 rAT-TK1a 3.55 244 0.106 29900 rHSV1-TK 1.7 4.1 0.028 16500

Tables 6 and 7 demonstrate that plant TK's phosphorylate AZT more efficiently (k_(cat)/K_(m)) than Thd, which is in sharp contrast to the rHSV1-TK which phosphorylates Thd far more efficiently than AZT. The ratio [k_(cat)/K_(m) (Thd)]/[k_(cat)/K_(m) (AZT)] for the tomato, Arabidopsis and HSV1-TKs are as follows: rTOM-TK1 1.07 rAT-ΔN45TK1b 0.06 rAT-TK1a 0.08 rHSV1-TK 72.7

Example 4

Growth of Transformed E coli KY895 on Nucleoside Analog Plates

This example describes how host cells transformed with the plasmids obtained according to Example 2 are able to grow on plates in presence of the nucleoside analog AZT (3′-azido-3′-deoxythymidine).

The experiment was carried out as described by Knecht et al. [Knecht, W., Munch-Petersen, B. & Pi{hacek over (s)}kur, J.: Identification of residues involved in the specificity and regulation of the highly efficient multisubstrate deoxyribonucleoside kinase from Drosophila melanogaster; J. Mol. Biol. 2000 301 827-837]. TABLE 8 Growth of KY895 Cells in Presence of AZT KY895 transformed with LD₁₀₀ (μM) pGEX-2T >100 pGEX-2T-TOM-TK1 0.0316 pGEX-2T-AT-TK1a 0.316 pGEX-2T-AT-TK1b 1 pGEX-2T-AT-ΔN22TK1b 0.316 pGEX-2T-AT-ΔN45TK1b 0.1 pGEX-2T-AT-ΔN63TK1b 0.1 pGEX-2T-HSV1-TK 1 pGEX-2T-hu-TK1 1 pGEX-2T-Bm-dNK 100 pGEX-2T-Dm-dNK 100 pGEX-2T is the vector and is available from Amersham-Pharmacia; pGEX-2T-Dm-dNK is the vector containing the gene encoding a multisubstrate deoxyribonucleoside kinase derived # from Drosophila melanogaster described by Munch-Petersen et al. [Munch-Petersen B, Knecht W, Lenz C, SØndergaard L, # Piskur J: Functional expression of a multisubstrate deoxyribonucleoside kinase from Drosophila melanogaster and its C-terminal deletion # mutants; J. Biol. Chem. 2000 275 6673-6679]; pGEX-2T-Bm-dNK is the vector containing the gene encoding a deoxyribonucleoside kinase derived from Bombyx mori described by Knecht et al. # [Knecht W, Ebert Petersen G, Munch-Petersen B, Piskur J: Deoxyribonucleoside kinases belonging to the thymidine kinase 2 (TK2)-like group vary significantly in # substrate specificity, kinetics and feed-back regulation; J. Mol. Biol. 2002 315 529-540]; pGEX-2T-huTK1 is the vector containing the gene encoding a human thymidine kinase (TK1) described by Berenstein et al. [Berenstein D, Christensen J F, Kristensen T, # Hofbauer R, Munch-Petersen B: Valine, not methionine, is amino acid 106 in human cytosolic thymidine kinase (TK1). Impact on oligomerization, stability, and kinetic properties; # J. Biol. Chem. 2000 275 (41) 32187-32192].

Table 8 shows that the thymidine kinase devired from tomato is the mose potent kinase when determined in combination with the nucleoside analog AZT. This enzyme is about 30 times as active as that derived from Herpes simplex.

Example 5

Testing for TK Activity on TK Selection Plates

This example describes determination of thymidine kinase activity using KY895 cells transformed with the plasmids described in Example 2.

The test for thymidine kinase activity was carried out as described by Knecht et al. [Knecht, W, Munch-Petersen, B. & Pi{hacek over (s)}kur, J.: Identification of residues involved in the specificity and regulation of the highly efficient multisubstrate deoxyribonucleoside kinase from Drosophila melanogaster; J. Mol. Biol. 2000 301 827-837].

pGEX-2T is the vector available from Amersham-Pharmacia used as control.

The multisubstrate deoxyribonucleoside kinase from the fruiffly Drosophila melanogaster (pGEX-2T-Dm-dNK), obtained according to Munch-Petersen et al. [Munch-Petersen B, Knecht W. Lenz C, Søndergaard L, Pi{hacek over (s)}kur J: Functional expression of a multisubstrate deoxyribonucleoside kinase from Drosophila melanogaster and its C-terminal deletion mutants; J. Biol. Chem. 2000 275 6673-6679] was used as a positive control. TABLE 9 Growth on TK Selection Plates Thymidine (μg/ml) 0.05 1 2 10 20 50 pGEX-2T − − − − − − pGEX-2T-Dm-dNK + + + + + + pGEX-2T-TOM-TK1 + + + + + + pGEX-2T-Rice-TK1 n.d. n.d. + n.d. n.d. n.d. pGEX-2T-AT-TK1a n.d. n.d. + n.d. n.d. n.d. + growth − no growth n.d. not determined

This example demonstrates that all enzymes are able to phosphorylate thymidine.

Example 6

Phosphorylation of Nucleoside Monophosphates to Nucleoside Diphosphates

Monophosphate kinase activity was tested by determining the products of the thymidine kinase catalyzed reactions, essentially as described by Munch-Petersen et al.; J. Biol. Chem. 1998 273 7 3926-3931.

Briefly, the amount of enzyme catalyzing the conversion of 30 nmol/min of thymidine to thymidine monophosphate (30 mU) was incubated in 100 μl of a mixture of 50 mM Tris-HCl, pH 8.0 (22° C.); 2.5 mM MgCl₂; 2.5 mM ATP; 10 mM dithiothreitol; 0.5 mM CHAPS; 3 mg/ml of bovine serum albumin; and 100 μM 3H-labelled substrate (1.8 Ci/mmol).

Time samples of 5 μl reaction mixture were mixed with 5 μl 5 mM thymidine, TMP, TDP and TTP. 5 μl, of this mixture were plated on polyethylenimine-cellulose plates, which were developed ascending in 0.5 M LiCl₂. The spots with the nucleosides and nucleotides were identified under UV light and cut out.

The radioactivity in the spots was extracted with 0.2 M KCl in 0.1 M HCl, and determined by liquid scintillation counting. 1 pmol radioactive nucleoside/tide counts 850 cpm.

Recombinant thymidine kinases were expressed and purified as described in Example 3.

The results of this experiment are presented in FIGS. 1 and 2. The cpm obtained at the indicated times (15, 30, 60, 90 and 120 minutes, respectively) are determined in the TDP spots derived from 2.5 μl of the reaction mixture.

As expected, HSV1-TK phosphorylates both substrates, but the AZT-monophosphate substrate is phospholylated at a 10 fold lower degree than the thymidine substrate. Surprisingly, however, is that the plant TK1 enzymes are capable of phosphorylating AZT-monophosphate although they are unable to phosphorylate TMP. This new property has never before been reported for any thymidine kinase. 

1. A pharmaceutical composition comprising a pharmaceutically acceptable carrier or diluent and an isolated polynucleotide, a vector comprising said polynucleotide and a promoter, a packaging cell line capable of producing an infective virion comprising said vector, or a host cell comprising said vector, said isolated polynucleotide encoding a plant thymidine kinase enzyme derived from a plant, said isolated polynucleotide being selected from the group consisting of: i) an isolated polynucleotide encoding thymidine kinase enzyme derived from thale cress (Arabidopsis thaliana), pine (Pinus taeda), rice (Oryza sativa) or tomato (Lycopersicum esculentum), ii) an isolated polynucleotide encoding a thymidine kinase enzyme derived from a plant, which thymidine kinase enzyme, when compared to human Herpes simplex virus type 1 thymidine kinase (HSV1-TK) and upon transduction into a eukaryotic cell, decreases at least four (4) fold the IC₅₀ of at least one nucleoside analogue, iii) an isolated polynucleotide encoding a thymidine kinase enzyme derived from a plant, which thymidine kinase enzyme is capable of phosphorylating at least one nucleoside analogue monophosphate at a higher degree than thymidine monophosphate, iv) an isolated polynucleotide encoding a thymidine kinase enzyme derived from a plant, which thymidine kinase enzyme has a ratio of [k_(cat)/K_(m) (Thd)]/[k_(cat)/K_(m) (at least nucleoside analogue)] of less than two (2), v) an isolated polynucleotide encoding a thymidine kinase enzyme derived from a plant, which thymidine kinase enzyme, when expressed and compared to human Herpes simplex virus type 1 thymidine kinase (HSV1-TK), has a decreased ratio of [k_(cat)/K_(m) (Thd)]/[k_(cat)/K_(m) (at least one nucleoside analogue)] of at least five (5) fold, wherein analogue is any nucleoside analogue, and vi) an isolated mutated and/or truncated polynucleotide encoding a thymidine kinase enzyme variant derived from a plant, which thymidine kinase enzyme variant, when compared to human Herpes simplex virus type 1 thymidine kinase (HSV1-TK) and upon transduction into a eukaryotic cell, decreases at least four (4) fold the IC₅₀ of at least one nucleoside analogue.
 2. The pharmaceutical composition of claim 1, wherein the at least one nucleoside analogue monophosphate is AZT monophosphate and/or wherein the at least one nucleoside analogue is AZT.
 3. The pharmaceutical composition of claim 1, wherein said polynucleotide is capable of hybridising under at least medium stringency conditions with the polynucleotide sequence presented as SEQ ID NO: 1, as SEQ ID NO: 3, as SEQ ID NO: 6, as SEQ ID NO: 8, as SEQ ID NO: 10, as SEQ ID NO: 12, or its complementary strand.
 4. The pharmaceutical composition of claim 3, wherein said polynucleotide has at least 73%, identity to the polynucleotide sequence presented as SEQ ID NO: 1, as SEQ ID NO: 3, as SEQ ID NO: 6, as SEQ ID NO: 8, as SEQ ID NO: 10, or as SEQ ID NO: 12, when determined over its entire length.
 5. The pharmaceutical composition of claim 1, wherein said polynucleotide comprises the polynucleotide sequence presented as SEQ ID NO: 1, as SEQ ID NO: 3, as SEQ ID NO: 6, as SEQ ID NO: 8, as SEQ ID NO: 10, as SEQ ID NO: 12, or a functional analog thereof.
 6. A pharmaceutical composition comprising a pharmaceutically acceptable carrier or diluent and an isolated plant thymidine kinase enzyme selected from the group consisting of: i) an isolated thymidine kinase encoded by the polynucleotide of claim 1, ii) an isolated plant thymidine kinase enzyme derived from thale cress (Arabidopsis thaliana), pine (Pinus taeda), tomato (Lycopersicum esculentum) or rice (Oryza sativa), iii) an isolated plant thymidine kinase enzyme, which thymidine kinase enzyme, when compared to human Herpes simplex virus type 1 thymidine kinase (HSV1-TK) in a eukaryotic cell, decreases at least four (4) fold the IC₅₀ of at least one nucleoside analogue, iv) an isolated thymidine kinase enzyme, derived from a plant, which thymidine kinase enzyme is capable of phosphorylating at least one nucleoside analogue monophosphate at a higher degree than thymidine monophosphate, v) an isolated thymidine kinase enzyme derived from a plant, which thymidine kinase enzyme has a ratio of [k_(cat)/K_(m) (Thd)]/[k_(cat)/K_(m) (at least one nucleoside analogue)] of less than two (2), vi) an isolated thymidine kinase enzyme derived from a plant, which thymidine kinase enzyme, when expressed and compared to human Herpes simplex virus type 1 thymidine kinase (HSV1-TK), has a decreased ratio of (k_(cat)/K_(m) (Thd)]/[k_(cat)/K_(m) (at least one nucleoside analogue)] of at least five (5) fold, wherein analogue is any nucleoside analogue, and vii) an isolated mutated and/or truncated thymidine kinase enzyme variant derived from a plant, which thymidine kinase enzyme variant, when compared to human Herpes simplex virus type 1 thymidine kinase (HSV1-TK) and upon transduction into a eukaryotic cell, decreases at least four (4) fold the IC₅₀ of at least one nucleoside analogue.
 7. The pharmaceutical composition of claim 6, wherein the at least one nucleoside analogue monophosphate is AZT monophosphate and/or wherein the at least one nucleoside analogue is AZT.
 8. The pharmaceutical composition of claim 6, wherein said thymidine kinase comprises one or more of the following three motifs/regions: Val Ile Gly Ile Asp Glu Ala Gln Phe (Motif I) Phe Val Ala Gly Leu Asp Gly (Motif II) Tyr Met Pro Val Cys Arg (Motif III) Val A1 Lys Leu A2 A3 Arg Cys Glu A4 (Lid region),

wherein A1 is selected from Thr and Val, A2 is selected from Thr and Lys, A3 is selected from Ala and Ser, and A4 is selected from Leu and Val.
 9. The pharmaceutical composition of claim 6, wherein said thymidine kinase comprises all of the conserved residues identified in Table
 1. 10. The pharmaceutical composition of claim 6, wherein said thymidine kinase comprises the amino acid sequence of SEQ ID NO: 2, SEQ ID NO: 4, SEQ ID NO: 5, SEQ ID NO: 7, SEQ ID NO: 9, SEQ ID NO: 11 or SEQ ID NO: 13, or an amino acid sequence of at least 70% identity with any one of these sequences, when determined over its entire length.
 11. The pharmaceutical composition of claim 6, wherein said thymidine kinase comprises the amino acid sequence of SEQ ID NO: 2, SEQ ID NO: 4, SEQ ID NO: 5, SEQ ID NO: 7, SEQ ID NO: 9, SEQ ID NO: 11 or SEQ ID NO: 13, or a functional analogue thereof.
 12. The pharmaceutical composition of claim 6, wherein said thymidine kinase has a C-terminal deletion in the order of 1-60 amino acid residues.
 13. The pharmaceutical composition of claim 12, wherein said thymidine kinase enzyme derived from tomato and having a C-terminal deletion of 26 amino acid residues (having the amino acid sequence of SEQ ID NO: 5).
 14. The pharmaceutical composition of claim 12, wherein said thymidine kinase enzyme is derived from thale cress and having a C-terminal deletion of 24 amino acid residues (having the amino acid sequence of SEQ ID NO: 11).
 15. The pharmaceutical composition of claim 6, wherein said thymidine kinase is derived from pine, and which enzyme shows at least 60% identity with the amino acid sequence presented as SEQ ID NO:
 2. 16. The pharmaceutical composition of claim 6, wherein said thymidine kinase is derived from tomato, and which enzyme shows at least 70% identity with the amino acid sequence presented as SEQ ID NO:
 4. 17. The pharmaceutical composition of claim 6, wherein said thymidine kinase is derived from rice, and which enzyme shows at least 70% identity with the amino acid sequence presented as SEQ ID NO:
 7. 18. The pharmaceutical composition of claim 6, wherein said thymidine kinase is derived from thale cress, and which enzyme shows at least 70% identity with the amino acid sequence presented as SEQ ID NO:
 9. 19. The pharmaceutical composition of claim 6, wherein said thymidine kinse is derived from thale cress, and which enzyme shows at least 70% identity with the amino acid sequence presented as SEQ ID NO:
 11. 20. The pharmaceutical composition of claim 6, wherein said thymidine kinase is derived from thale cress, and which enzyme shows at least 70% identity with the amino acid sequence presented as SEQ ID NO:
 13. 21. The pharmaceutical composition of claim 6, wherein said thymidine kinase decreases at least three (3) fold the lethal dose (LD₁₀₀) of at least one nucleoside analogue when compared to the action of a thymidine kinase derived from human Herpes simplex virus type 1 thymidine kinase (HSV1-TK).
 22. A method of sensitising a cell to a prodrug, which method comprises the steps of (i) transfecting or transducing said cell with a polynucleotide sequence encoding a plant thymidine kinase enzyme that promotes the conversion of said prodrug into a (cytotoxic) drug; and (ii) delivering said prodrug to said cell; wherein said cell is more sensitive to said (cytotoxic) drug than to said prodrug.
 23. The method of claim 22, wherein the polynucleotide sequence encoding a plant thymidine kinase enzyme is a polynucleotide sequence of claim
 1. 24. The method of claim 22, wherein the prodrug is a nucleoside analogue.
 25. The method of claim 24, wherein the nucleoside analogue is AZT (3′-azido-3′-deoxythymidine).
 26. A method of inhibiting a pathogenic agent in a warm-blooded animal, which method comprises administering to said animal a polynucleotide or a vector of claim
 1. 27. The method of claim 26, wherein said polynucleotide sequence or said vector is administered in vivo.
 28. The method of claim 26, wherein said pathogenic agent is a virus, a bacteria or a parasite.
 29. The method of claim 26, wherein said pathogenic agent is a tumour cell.
 30. The method of claim 26, wherein said pathogenic agent is an autoreactive immune cell.
 31. The method of any claim 26, further comprising the step of administering a nucleoside analogue to said warm-blooded animal.
 32. The method of claim 31, wherein said nucleoside analogue is AZT (3′-azido-3′-deoxythymidine).
 33. (canceled)
 34. A method of phosphorylating a nucleoside analog monophosphate, comprising the steps of i) subjecting the nucleoside analog monophosphate to the action of the plant thymidine kinase enzyme of claim 6, and ii) recovering the nucleoside analog diphosphate.
 35. A method of controlling or modifying growth of a plant, which plant comprises plant cells comprising a polynucleotide encoding a plant thymidine kinase enzyme of claim 6, which method comprises the step of exposing the plant or plant cell to a nucleoside or nucleoside analog.
 36. Articles containing a nucleoside analogue and the thymidine kinase according to claim 6, or a gene coding for said plant derived thymidine kinase, or vector comprising said gene coding for said plant derived thymidine kinase as a combination for the simultaneous, separate or successive administration in cancer therapy.
 37. An isolated plant thymidine kinase enzyme derived from pine having at least 70% sequence identity to the sequence presented in SEQ ID NO:
 2. 38. An isolated plant thymidine kinase enzyme derived from tomato having at least 70% sequence identity to any of the sequences SEQ ID NO: 4 or
 5. 39. An isolated plant thymidine kinase enzyme derived from Arabidopsis thaliana having at least 70% sequence identity to the sequence SEQ ID NO: 9 or
 11. 